1. Field of the Invention
The present invention relates to a process for preparing semiconductor devices. More particularly, it relates to a process for preparing a plurality of semiconductor pellets from one sheet of a semiconductor wafer.
2. Description of the Prior Art
It has been well known to prepare many semiconductor pellets as semiconductor devices in one sheet of a semiconductor wafer in a preparation of semiconductor elements for semiconductor devices. This process is remarkably effective for lowering a processing cost especially in planar type semiconductor devices requiring many steps in a processing of a wafer and glass passivation mesa type semiconductor devices forming a protection of junction exposed surface in the form of a wafer.
FIG. 1 is a partially broken schematic view of a semiconductor wafer structure forming many mesa type thyristor devices which has a surface protection, wherein a p-type anode layer (3) is formed on one surface of the n-type substrate (2) of a semiconductor wafer (1) and a p-type base layer (4) is formed on the rare surface and n-type cathode layers (5) which are separated for each of thyristor pellets E are formed in the p-type base layer (4), whereby each pellet E has pnpn structure and junctions J.sub.1, J.sub.2 and J.sub.3 are formed between the layers.
The pellets E formed in the semiconductor wafer (1) are respectively separated by grooves (6) shaved on both surfaces of the wafer (1) between them. The grooves are called as mesa grooves and shaved in deeper than the junctions J.sub.1 and J.sub.2 to separate the p-type anode layer (3), p-type base layer (4), n-type cathode layers (5) and the junctions J.sub.1, J.sub.2 and J.sub.3 for the pellets, however, the n-type substrate (2) for the n-type base layer is not separated to maintain the shape of the wafer (1). In the condition, the surface protective material (7) is applied in the mesa grooves (6) to electrically and mechanically protect and stabilize the junctions exposed in the mesa grooves (6). The surface protective material (7) can be silicon oxide film, silicone rubber and frit. The semiconductor wafer (1) having the surface protective material is divided along the mesa grooves (6) to form thyristor pellets E.
In accordance with the process for preparing the semiconductor pellets, the surface protective treatment can be applied simultaneously for many semiconductor pellets in a form of a wafer and the processing efficiency is higher and the processing cost for preparing semiconductor devices can be remarkably lowered in comparison with the process for applying the protective surface treatment on the surfaces of pellets after separating the wafer into pellets in the step of forming pn-junctions. It is possible to simultaneously attain the diffusion and the formation of electrodes by one step even in the case of planar type devices. Accordingly this process for preparing pellets is remarkably effective.
A number of pellets formed in one sheet of a wafer is increased by increasing the area of the semiconductor wafer to prepare more pellets by one operation and to decrease the processing cost. Accordingly, it is preferable to use a larger wafer. However, when a diameter of the wafer is larger, the wafer is easily broken in the process for preparing pellets and the processing cost disadvantageously increases. When the semiconductor wafer having the mesa grooves is processed, for example, frit is applied in the mesa grooves having a depth of 50 to 70.mu. formed on the semiconductor wafer having a diameter of 40 mm, it is necessary to use the wafer having a thickness of more than 220-240.mu. in order to prevent the break of the wafer because the thermal expansion coefficient of the frit is higher than that of the silicon to cause the strain in the calcination of the frit. The wafer having a diameter requires a thickness of more than 250-270.mu. and the wafer having a diameter of 75 mm requires a thickness of more than 330-350.mu..
When the planar type devices are formed on a semiconductor wafer, the wafer having a diameter of 40 mm requires a thickness of more than 180.mu.; the wafer having a diameter of 50 mm requires a thickness of more than 220.mu.; the wafer having a diameter of 65 mm requires a thickness of more than 270.mu. and the wafer having a diameter of 75 mm requires a thickness of more than 300.mu..
From the viewpoint of characteristics of the semiconductor device, the increase of the thickness of the wafer causes the increase of forward voltage drop or the decrease of over current endurance and the substantial decrease of current capacity, disadvantageously, in the case of devices for passing current in the direction perpendicular to the wafer surface such as diodes and thyristors. Accordingly, the thickness of the wafer is limited from this viewpoint. From the viewpoint of the other characteristics, a thyristor having a withstand voltage of 500-600 V requires a thickness of the wafer of 180 to 220.mu.. When the thickness of the wafer is more than 220.mu., the forward voltage drop disadvantageously increases. Accordingly, it has been difficult to use a wafer having a diameter of more than 50 mm for a preparation of thyristors.